Restarting a Tank

[WAQuitta]

I was gifted a roughly 25 gallon hex tank. We kept the set up about the same. We are now ready to change it up some. I really need advise on what kind of filter to use and what else I need. The original top is missing so it was replaced with a piece of Plexiglas, but the water evaporates quickly. Any ideas on how to replace. We have tropical fish now and are going to continue with them. Any help would be most appreciated.